The invention relates to the field of steering control systems fitted to motor vehicles, and particularly industrial vehicles, such as trucks. It relates more specifically to steering systems in which the turning of the wheels is not obtained by a mechanical transmission but by means of devices transmitting electrically or even hydraulically the instructions given by the driver. These steering systems are known under the general name of “Steer by Wire”. It is desirable to provide an enhancement of the command and control method of certain types of “Steer by Wire” device.
Usually, steering systems of the “Steer by Wire” type are more particularly designed taking account of the concerns relating to reliability, since the steering is a safety function, and these systems use many electronic components carrying out numerous complex computations.
Therefore, it has already been proposed to duplicate the electrohydraulic circuits and the associated control, for the purpose of redundancy, and in order to supplant one circuit by the other when one of them becomes faulty. Therefore, document WO 2004/009425 describes a steering control system which comprises two electrohydraulic circuits operating simultaneously. Each of these circuits includes an actuator acting on the turning angle of the wheels. The two actuators are mechanically coupled so that, if one of the electrohydraulic circuits (or the associated command?) fails, the other circuit takes over.
Because the two actuators are mechanically coupled, it is necessary to use different control strategies. Therefore, a first actuator is displacement-controlled, while the second is stress-controlled. Therefore, when the circuits are designed in a similar manner, each supplies substantially half of the power necessary to turn the wheels. However, instability phenomena may manifest themselves when the controls of the two circuits are not exactly synchronized, in particular because of different reaction time and inertia of one circuit versus the other.
Therefore, in the particular case in which the hydraulic circuit includes “closed-center” solenoid valves, particular difficulties may arise.
Specifically, closed-center valves are such that, at rest, they do not allow a fluid to pass through to the actuator. Therefore, because the actuators are mechanically coupled, the beginning of the opening of a solenoid valve does not allow a fluid to pass if the complementary solenoid valve of the other circuit is not also in the same state of opening.
In other words, if one solenoid valve begins to open while the other still remains closed for a fraction of a second, the fluid cannot flow and everything happens as if the solenoid valve that is already open had remained closed. It is conceivable therefore that these actuation differences may cause steering difficulties, or even instabilities that are prejudicial to correct operation of the system. FIG. 4a depicts a timing chart showing on a standard scale the change over time of the turning angle of the wheels, represented by the curve 141, in response to an instruction corresponding to a rotation of the steering wheel, represented by the curve 142. FIG. 4b illustrates, on a standard scale, the displacement of the solenoid valve slides of the valves of the two differently-controlled circuits. For the displacement-controlled circuit (curve 143), note that the amplitude of displacement of the solenoid valve slide is slightly ahead, and of greater amplitude than the displacement (curve 144) of the solenoid valve slide of the stress-controlled circuit. FIG. 4c illustrates the stress values evaluated for each circuit. Note that, surprisingly, at the beginning of the response, the stress (curve 146) applied in the stress-controlled circuit is not equal, but opposite to the stress (curve 147) exerted in the displacement-controlled circuit. In other words, because of the imperfections of this control method, the stress-controlled circuit that is supposed to assist the displacement-controlled circuit does not fulfill this function, but on the contrary sustains a stress from the position-controlled circuit until a nominal operation is achieved. The result of this is an additional consumption of power that is not converted into kinetic energy.
A difference in the command instructions of the two solenoid valves is virtually inevitable because the two controls operate on variables of different kinds, measured by sensors of a different type.
Specifically, the displacement-controlled circuit uses a distance sensor, measuring the travel made by the actuator. Conversely, the stress-controlled circuit uses several pressure sensors, mounted on the various chambers of the two actuators. The pressure sensors being of a different design from the displacement sensors, it can be seen that they generate a difference of reaction time of the two control circuits.
The invention therefore relates to a steering system controlling the turning angle of the steering wheels of a vehicle. In a known manner, such a system comprises two electrohydraulic circuits each including an actuator. The two actuators are mechanically coupled and act simultaneously on the turning angle of the wheels. A first of the two circuits is movement-controlled relative to a position instruction, while the second circuit is stress-controlled.
According to the invention, this system is characterized in that the instruction applied to the second circuit comprises a component developed from a filtering of the position instruction applied to the first circuit.
In other words, the circuit that is stress-controlled also reacts to the position command, for transitional phases, making it possible to prevent locking phenomena.
The operation of the second circuit is therefore not simply the result of a stress control, but takes account partially of the displacement command which acts on the other circuit. Therefore, since the stress control is usually slower than the displacement control, the addition of this additional characteristic component makes it possible to anticipate the movements of its solenoid valves, relative to a configuration of the prior art, and notably that described in document WO 2004/09425.
In practice, the characteristic component developed from the filtering of the position instruction may include a filtering of the high-pass type. In other words, the injection of this characteristic component makes it possible to cause the stress-control to react rapidly to the changes of the displacement instruction. In other words, the invention therefore allows the stress-controlled circuit to see its solenoid valves move virtually as quickly as those of the displacement-controlled circuit.
Advantageously in practice, the stress-control of the second circuit may include a low-pass filtering of the stress measurement made on the actuator or actuators, so as to decouple in frequency the influence of the characteristic additional component of the main stress-control component. In other words, for rapid turning movements, the stress-controlled circuit reacts mainly thanks to the additional component based on the position instruction.
Conversely, in a low-frequency band, for virtually static movements, the mainly stress-controlled circuit is no longer sensitive to the position command, but to its main loop interfaced with the pressure sensor.
Such a control architecture makes it possible to compensate for the harmful time-lags mentioned for the circuits using closed-center valves. However, it may also have a response-enhancement value for circuits using open-center valves.
Advantageously in practice, each electrohydraulic circuit may be controlled by a command and control unit that is specific thereto, for the purpose of redundancy and of increasing reliability.
Advantageously in practice, the position instruction may be developed from a steering control member on which the driver takes action.
In practice, the system may comprise at least one sensor measuring the displacement of one or other of the actuators, and several sensors measuring the pressures prevailing in the chambers of the actuators.
According to a first embodiment, the position instruction may be transmitted to the electronic command and control units each assigned to an electrohydraulic circuit. In this case, advantageously, the signals originating from the sensor or sensors measuring the displacement of one or other of the actuators are conveyed to the two electronic command and control units assigned to each of the electrohydraulic circuits.
In another embodiment, the signals originating from the sensor or sensors measuring the displacement of one or other of the actuators are conveyed to the electronic command and control unit assigned to the electrohydraulic circuit operating with the displacement-control, said unit transmitting to the other unit a signal representative of the difference between the position instruction and the displacement measurement.
Therefore, depending on the case, the component applied to the second circuit and which is developed from the filtering of the position instruction may take account directly of the same displacement instruction as that which is used for the first circuit. But it is also possible that it is developed from a deviation signal used for the displacement control of the first circuit, that is to say indirectly from the displacement instruction, from which the effective displacement measurement has been subtracted.